Application of visbreaker analysis tools to optimize performance

ABSTRACT

A system and method are outlined for controlling and optimizing chemical injection into a process unit to control fouling. The method uses an optical device to measure the fouling propensity of the process fluid at various points within the process unit. The measurements are compared with one another and prediction methods are used to evaluate the fouling potential within the unit, and determine the proper chemical dosage. Antifoulant chemical is then introduced into the unit to control the rate of fouling. The method and application continue on a frequent basis to maintain optimal fouling control within the unit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/456,128 filed Jul. 7, 2006, now allowed, which was, in turn,a continuation-in-part of U.S. patent application Ser. No. 11/178,846filed Jul. 11, 2005 now U.S. Pat. No. 7,394,545.

FIELD OF THE INVENTION

The present invention relates to systems and methods for characterizingand quantifying a dispersive medium; specifically, measuring theconcentration of particles or the tendency toward forming a dispersedphase within a fluid sample. The present invention also provides aprogram which uses these measurements of concentration to monitor andcontrol operation of a process unit in a refinery.

BACKGROUND OF THE INVENTION

Thermal conversion is a process in which, by the application of heat,large hydrocarbon molecules are broken into smaller molecules with alower boiling point. These operations are carried out in the industry ofcrude oil refining by plants such as a visbreaker, coker, andhydrocracker for obtaining intermediate or light cuts of higher value,from heavy residues of lower commercial value. As is evidenced by EPPat. No. 768363, the term “process unit” can also be used in the placeof “plant”. The thermal cracking applied in the visbreaker process willalso reduce the viscosity and pour point of the heavy residues.

It is well known that the fouling potential of a fluid can be estimatedand characterized by the concentration of the dispersed phase,particularly by the concentration of the dispersed phase present in aspecific size range. In hydrocarbon systems in particular, it has beenrecognized that the concentration of asphaltenes (i.e., carbon particlesor opaque species) with linear dimension greater than about 2 microns invisbroken tars is a good indication of the fouling potential of thematerial.

The VSB process was developed some years ago with the intention ofobtaining a viscosity decrease in heavy products in order to reduce theamount of higher valued flux to meet the viscosity specification of thefinished heavy fuel product. Today, however, it is managed withsubstantially different objects, namely with the aim of obtaining amaximum transformation into middle and light distillates to meet themarket requirements.

The controlling factor in obtaining a high conversion is the need toobtain a stable residue. In fact an increase of the cracking temperaturecertainly would involve a higher conversion in light and middledistillates, but it would produce a much more instable tar which wouldproduce a final product outside the required stability specifications.

An increase of the light streams is achieved by increasing the crackingseverity through an increase of the outlet furnace temperature of theVisbreaker furnace. While increasing this temperature arbitrarily willserve to drive the conversion rate higher, it also comes at the cost ofproducing a highly unstable tar as a precipitate in the process, with ahigh concentration of asphaltene particulates. This particulate matterconstitutes a severe fouling threat to the energy recovery devices(i.e., furnace and heat exchangers) in the process. As such, in order tomaximize the profitability a Visbreaker unit, it is desirable tooptimize the outlet furnace temperature while maintaining the stabilityof the produced tar. While it is known that high temperature dispersantsand anti-foulants can be introduced into the system to reduce thetendency and rate of fouling, prior art systems have not been entirelysatisfactory in providing an automated system for determining an optimumtype and/or quantity of chemical dispersants and anti-foulants to beintroduced into the visbreaker unit in order to maximize plantprofitability. The present teachings will show that if the foulingpotential of the tar can be quantified, then the precise level ofchemical inhibitor can be dosed to maximize the plant profitability.

It is also known that the fouling tendency of a hydrocarbon feedstock,or blends thereof, can be related to the tendency for insoluble organicmaterials to precipitate in heat transfer equipment, or other equipmentfor a refinery process unit.

Therefore, in one aspect the present invention provides a simplified,automated system and method that can easily be used to carry out opticalanalysis of visbroken tars and other fluid samples in order tocharacterize and quantify the concentration of particles within thefluid sample with high accuracy and reproducibility. In another aspect,the present invention utilizes these concentration measurements todetermine the fouling potential of the visbroken tars, and regulates theintroduction of chemical inhibitors into the visbreaker unit to improvethe yield of light streams. In yet another or further aspect, a sequenceof aliquots are prepared from the same sample at different dilutions todrive phase separation, producing a sequence of concentrationmeasurements correlated to a classical measurement of peptization value(PV), a qualitative measure of the product quality. In yet another orfurther aspect, this invention utilizes concentration data to estimatethe fouling potential of a feedstock, relates the concentration data tothe fouling potential of the feedstock, and provides optimal dosage ofchemical treatment to reduce the rate of fouling. These and otheraspects of the present invention will become apparent to those skilledin the art upon review of the following disclosure.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a system and method forestimating a concentration of inhomogeneities contained within a tarbyproduct of visbreaker operations. The invention does so by measuringthe modulation of transmitted light through a fluid sample. The systemuses a strongly convergent optical lens system to focus light onto aprepared sample. In one exemplary embodiment, the optics of aconventional optical microscope are used. A 3-dimensional translationstage is installed downstream of the focusing optics so that the samplecan be scanned over a large region, and at a specific focal plane. Aphoto detector is placed on the opposite side of the stage from thefocusing optics to measure the transmitted light through the sample. Thephotodetector is read-out by an analog-to-digital converter (ADC) inorder to provide a digital (i.e., quantitative) measure of thetransmitted light intensity. The translation stages are then moved in apattern, such that the intensity of the transmitted light is measuredover a representative path across the sample. When an opacity, scattereror opaque particle of a threshold size is encountered in the sample, theintensity of the transmitted light is strongly attenuated. Such changeof light intensity is then correlated with the detection of an opaqueparticle in order to characterize and quantify the concentration ofparticles within the fluid sample with high accuracy andreproducibility. Data processing algorithms are implemented to determinethe background noise level associated with the acquired data and to seta threshold level. As such, a specific signal-to-noise ratio can bespecified to define when a detection event is registered. Sizediscrimination may be achieved according to the physical dimensions ofthe beam waist of the focused light beam.

In another aspect, the present invention utilizes the concentrationmeasurement data to estimate the fouling potential of visbroken tars ina visbreaker unit in order to regulate introduction of chemicalinhibitors into the visbreaker unit and improve the yield of lightstreams. The invention provides an automated program which allows theuser to maximize the production of light streams by modeling thecorrelation between operational parameters such as feed quality,cracking severity, conversion rate, run length, and fouling rate of thesubject exchanger or furnace in order to regulate introduction ofchemical inhibitors into the visbreaker unit in accordance with customerspecifications and/or production requirements.

In another aspect, the present invention utilizes a method forcontrolling and optimizing chemical injection into a process unit tocontrol fouling. The method uses an optical device to measure thefouling propensity of the process fluid at various points within theprocess unit. The measurements are compared with one another andprediction methods are used to evaluate the fouling potential within theunit, and determine the proper chemical dosage. Antifoulant chemical isthen introduced into the unit to control the rate of fouling. The methodand application continue on a frequent basis to maintain optimal foulingcontrol within the unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the scanning apparatus of the present invention,showing the schematic relationship of the various elements;

FIG. 2 illustrates an example of a computer screen displaying a dataacquisition interface in accordance with the present invention;

FIG. 3 is a diagram illustrating optics used to convergently focus alight beam to a narrow beam waist;

FIG. 4 illustrates a plurality of spaced apart linear scans comparedwith a solid block representing an equivalent effective surface area;

FIG. 5 is a graph illustrating raw light transmission data obtained overa single line scan;

FIG. 6 is a graph illustrating the raw data of FIG. 5 after the data hasbeen filtered to remove line noise and gross intensity variations;

FIG. 7 is a graph illustrating decreasing statistical error as afunction of overall scan length;

FIG. 8 is a graph showing the correlation of sample inhomogeneity, asmeasured by the instrument to samples with a varying degree of dilutionfrom a fully cracked (i.e., high asphaltene particle density) sample;

FIG. 9 is a schematic of the mechanics of the chemical effect of thedispersants;

FIG. 10 is a graph of the relation of PV to the Furnace OutletTemperature (FOT) with and without chemical treatment;

FIG. 11 illustrates tar stability and conversion as asphaltenes aredisbursed in the continuous phase through the peptizing action ofaromatics and resins;

FIG. 12 is a graph illustrating raw data obtained from a visbreakerconversion trial;

FIGS. 13-16 are graphs illustrating raw data obtained from a conversionenhancement application;

FIG. 17 is a graph illustrating VFM data versus corr. skin temperature;

FIG. 18 is a schematic diagram illustrating exemplary visbreaker processtypes;

FIGS. 19A, 19B illustrate Pv measurement with a measurement system ofthe present invention;

FIG. 20 is a table illustrating VFM particle count data for aparticularly asphaltenic crude oil (Crude A) blended at several ratioswith a standard crude slate (Crude B);

FIG. 21 is a graph illustrating VFM particle count data for aparticularly asphaltenic crude oil (Crude A) blended at several ratioswith a standard crude slate (Crude B);

FIG. 22 is a graph illustration actual furnace inlet temperaturedeclines due to fouling as measured in the field;

FIG. 23 is a graph illustrating VFM particle count data for aparticularly asphaltenic crude (Crude D) blended into another standardcrude slate (Crude E); and

FIG. 24 is a schematic diagram illustrating how the VFM device can beused to control fouling in a process unit preheat exchanger train.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments and examples describing the present invention willbe described below with reference to the accompanying drawings. As shownin FIG. 1, this invention uses an optical system as generally indicatedby the number 10, which in the present exemplary embodiment comprises aconvergent lens, a light source 12, and a multi-axis translation stage14. The light source 12 may be implemented, for example, in the form ofa solid state visible laser. An infra-red (IR) laser may also be used,and is in some cases preferable owing to the fact that HC solutions aretypically much more transparent to IR light, than visible light. Thetranslation stage 14 may be moved horizontally in the x and y directionsin response to control signals generated by an associated computer 20 todirect the light beam along a plurality of paths through the sample. Thethird axis moves the stage vertically, towards and away from thefocusing lens. This permits selection of a focal plane within thesample. In another exemplary embodiment, the present inventioncontemplates providing means for moving the light source 12 with respectto the sample, thereby allowing the light beam to be directed throughthe sample to achieve the same results. Moreover, the present inventionalso contemplates usage of a flow cell to receive a flow of samplefluid, wherein the sample fluid flows through the flow cell while thelight beam is directed through a portion of the flowing sample. Alsoimplemented is a photodetector 16, for example, a PIN photodiode,located on the opposite side of the stage 14 to detect light 13 beingtransmitted through the sample volume, which is located on thetranslation stage. The photodetector 16, in turn, is connected by aconnector and cable 17, for example, a twisted pair with BNC connector,to an analog-to-digital (A/D) converter 18 to quantify the transmittedlight intensity. As described below, this is done to sample or detectthe occurrence of inhomogeneities in light transmission which may becaused by mineral and other inclusions, and agglomerating or stablelocalized dark matter of various types.

In one exemplary embodiment of the invention, a colloidal fluid samplematerial of thick viscous tar sampled from a Visbreaker is placed on thetranslation stage 14. Depending on the conditions in the Visbreakerunit, the sample may or may not contain asphaltene (or carbon based)particles. The asphaltene particles within the tar medium are opaque tovisible light. The tar medium is also opaque to visible light when thepath length through the medium typically exceeds a linear dimension ofabout 1 cm. A sample volume is dispensed on a slide, or flow cell 15such that a typical sample thickness of 10-20 microns is produced. Assuch, the thickness of sample medium should be made thin enough so as toprovide a differential transparency between the viscous tar medium andthe asphaltene particles in question. In this exemplary embodiment, inorder to optimize light transmission from a low power light source, asolid state laser that produces radiation at about 633 nm is chosen.This provides adequate power at a suitable region in the EM(electromagnetic) spectrum to provide transmission through a thin layerof tar, while the carbide particles remain opaque.

In order to have sensitivity to the specific sized inhomogeneities,appropriate optics should be used to focus the laser light onto thesample. The choice of a monochromatic light source allows the design ofthe optics to be optimized. As shown in FIG. 3, a highly convergent lenssystem 200 is used to focus the light beam 100 down to a beam waist ofapproximately 1 micron. The size of the beam waist determines theminimum cross-dimension an inhomogeneity must have to fully attenuatethe laser light. If an inhomogeneity is smaller than 1 micron, it willstill allow the transmission of light. As such, the focusing opticsdefine, in part, a threshold size for inhomogeneity detection. Anequation for calculating the beam waist is as follows:

W=0.61λ/d

Where

-   -   W=beam waist (1/e) width    -   λ=wavelength of light    -   d=numerical aperture

For example, if λ=633 nm and d=0.56, then W=0.7 μm.

Since we are interested in inhomogeneities larger than 1 micron (andsmaller than ˜20 microns), we do not use an IR laser, even though the HCsolutions are more transparent to IR radiation because the beam waistwould increase in size for the given optics. As such, we would reducethe sensitivity of the instrument. Preferably, the wavelength and beamwaist are also chosen to minimize interference artifacts that may ariseas the concentration of dispersed phase increases or the samplethickness varies (e.g., under a cover slide.)

The fluid sample 120 thickness is chosen to be about 10 microns. Thebeam 100 is focused on the slide 104, below a cover slip 102, or a flowcell in the sample volume. The depth and width of focus are constrainedby the optical system and the selected light wavelength. In oneexemplary embodiment, both dimensions are selected to be approximately 1micron.

FIG. 2 illustrates an example of a screen display presented by thesoftware of the present invention. The screen display illustrated inFIG. 2 represents a data acquisition interface allowing the operator tospecify a variety of scanning acquisition, analysis parameters,operating conditions of the instrument, and results of the measurement.The methods by which the operator selects items, inputs data, andotherwise interacts with the data acquisition interface areconventional, and further discussions of these operations are notprovided herein. In an exemplary embodiment of the invention, dataacquisition software was implemented via Visual Basic® in Excel® withanalysis and signal processing code being implemented in GNU Octave,although those skilled in the art of software programming willappreciate that many other software programming means may be used toachieve the same results.

A testing plan was designed and implemented to validate and measure thescanning performance of an exemplary embodiment of the presentinvention. In particular, measurement repeatability is validated byanalyzing the variation between identical measurements. Reproducibilityof the data is examined by analyzing the effects of scanning differentregions in the sample. This is complicated by the effects of sampleinhomogeneity. Accuracy of the system is tested by comparing thescanning data with visual images and PV (PV=peptization value) of thesample. Precision of results is analyzed for statistical uncertaintywith path length and by optimizing sample area, as discussed in moredetail below.

FIG. 4 illustrates an example of how the scanning system samples a largeregion of the sample. The array of linear scans (shown on the right sideof FIG. 4) represent the same effective surface area as the small boxillustrated on the left side of FIG. 4. For example, an array of 20linear scans of 15 mm length with a 1 micron wide laser beam effectivelysamples the same area as does the smaller 0.48 mm×0.64 mm box. However,by arranging the sampling path to extend over a larger region of thesample, the effects of sample inhomogeneity, local fluctuations in thesample, and sample variation are reduced drastically. As such, thestatistical results are much more accurate and reproducible.

To demonstrate the repeatability of our scanning results, five identical15 mm scans from a same sample, each covering a 0.015 mm² effective areawere measured. The measurement showed that the number of counts per 15mm line scan were identical within 95% confidence limits. Increasing thesampling region to 20-15 mm scan paths, the same systematic effects wereseen. After applying statistical analysis to the results, it wasobserved that the single line scan measurements are normallydistributed, with a standard deviation (σ)=1.6 counts on a mean of 8.0counts. Furthermore, the total integral count of the sample was 159 witha standard deviation of 9 counts. This shows that both the meaninhomogeneity count per path, and the total integral inhomogeneity countwere statistically identical and repeatable, over the separate trials,thus demonstrating that instrument stability and repeatability isexcellent. It also demonstrates that the fractional error can be reducedby increasing the sampling length. This is due to the fact thatindependent errors do not add linearly, but in quadrature.

As can be noted from the above data, the system of the present inventionis capable of minimizing and quantifying the effects of sampleinhomogeneity.

Turning now to FIG. 5, there is shown a graph representing exemplary rawdata obtained from a single line scan of 15 mm length taken during a 10second acquisition window.

In FIG. 6, the raw data of FIG. 5 is processed by a Fourier filtering toremove 50/60 Hz line noise and a median filter is used to remove grossintensity variations to extract the number of counts above a thresholdvalue. This process may be repeated for all line scans (e.g., 20 linescans) to calculate the total homogeneity areal density of the sampleunder test. In one example, the number of peak counts from a single linescan is calculated as

ρ₁=(9±3)÷(15 mm×1 μm)=600±200 mm⁻²

Repeating this calculation for a measurement spanning over 20 paths, theerror decreases as shown below:

ρ_(tot)=(149±12)÷(20×15 mm×1 μm)=497±40 mm²

We see that the error decreases according to Gaussian statistics wherethe error propagates in quadrature which is a well known statisticalproperty.

As shown in FIG. 7, an approximate 5% uncertainty is achieved at 10 linescans of 15 mm length (i.e., 0.15 mm² effective area). Statistical erroris thus shown to decrease with N^(−0.6), where N is the number of 15 mmpath length multiples. From the exemplary data of FIG. 7, it is shownthat an overall path length of about 150 mm (10×15 mm) would achieve anapproximate 5% error.

In order to determine the background noise in the signal as in FIGS. 5and 6, the present invention provides a software algorithm, whichautomatically computes the background noise and sets a discriminatorlevel or threshold for registering a sample inhomogeneity. A measurementof the light transmission is made when no scanning is occurring. Thus,the signal is an estimate of the nominal noise. Calculating the standarddeviation of this signal distribution allows the estimate. The value canbe used to determine a fixed signal-to-noise ratio on which to acceptinhomogeneities.

In accordance with the present invention, the instrument is capable ofquantifying the inhomogeneity of a solution in an automated and timelyfashion.

To demonstrate the capabilities of the present invention, the followingsample specimens, with various concentrations of asphaltenes were usedfor analysis and validation:

-   -   Specimen A: 9630 Asls, PV=1.7, low particulate density (highly        diluted).    -   Specimen B: 9630-6, PV=1.4, intermediate particulate density        (partially diluted).    -   Specimen C: 9630-7, PV<1.0, high particulate density, heavily        cracked sample (slightly diluted).    -   Specimen D: 9630-mod, 13% 9630-7+9630 Asls, PV=about 1.35        (partially diluted).

The scanning results from these samples were then compared tophotographs of the samples, and a correlation was found between theimages and the scanned results. A graph showing the correlation ofparticle density as measured by the instrument to samples with a varyingdegree of dilution from a fully cracked (i.e., high asphaltene density)is shown in FIG. 8.

Overall, the testing results demonstrate that the system of the presentinvention provides good repeatability and shows correlation with visualimage views. It has been shown that a relatively large sample area maybe covered with automated operation, thus reducing the effects of localfluctuations in inhomogeneity density. Data can also be assigned anerror to quantify precision of results.

We also disclose a program to monitor and control the operation of aVisbreaker unit in a hydrocarbon processing facility (refinery). Theprogram allows the user to maximize the production of light streams(i.e., usually diesel) while maintaining a highly stable residual tarand reducing the chance that rundown of the tar will foul the preheatheat exchangers.

It is known that the stability of residual visbroken tar and its foulingpotential can be measured by the peptization value (PV) and the hotfilterable solids (HFT). Note that HFT and PV are two different metricsas HFT is a product specification whereas PV is a characterization ofthe visbroken tars towards the asphaltene precipitation potential. Theoptical measurement device (referred to hereinafter as ‘VFM’) of thepresent invention measures a quantity which is a measure of the opaquefilterable solids within a tar sample. The automated program of thepresent invention utilizes the VFM concentration measurement data toestimate the fouling potential of the visbroken tars. This estimate inturn is used to gauge the needs for optimum feed of chemical treatments.

It is known that high temperature dispersants and anti-foulants are themain components in a chemical regiment used to treat Visbreakers. Thereare specific chemical families that are particularly effective for usein the Visbreaker for reducing fouling of heat exchanging surfaces(i.e., exchanger, furnace, etc.) and subsequently stabilizing theproduced visbroken tar. The program of the present invention isconfigured to select the type and quantity of chemistry required tosatisfy production requirements. Specific chemical entities include, butare not limited to polyisobutenylphosphonic acids and esters,polyisobutenylthiophosphonic acids and esters, alkylphosphonate phenatesulfides and disulfides that may be neutralized with alkaline earthmetals or amines polyisobutenyl succinimides, polyisobutenylsuccinatealkyl esters, magnesium or calcium salts of alkyl or dialkylnaphthelenesulfonic acids as described in U.S. Pat. No. 4,927,519 and EP Patent No.321424B1.

These antifoulant materials have been found to function at low dosages,1-200 ppm, to prevent the undesirable deposition or fouling of surfacesin visbreakers, as well as prevent the carboneceous deposition invisbroken heavy oil products (tar). Fouling in heat exchangers is mostgenerally thought to occur by first generating an unstabilizedmacromolecular particle that is no longer dissolved in the fluid, or isno longer a stable colloidal species. This occurs due to the thermalstress on the hydrocarbon. Initial deposition occurs, and furtherdestabilized species adsorb onto the site of original deposition. Biggerparticles in the hydrocarbon will be more prone to contact and coalesceto the surface. Dehydrogenation of the adsorbed hydrocarbon will bedriven by heat and make the deposit more tenacious as crosslinkingreactions occur.

The dispersants are generally understood to function by a variety ofmechanisms. First, the dispersant materials adsorb to the surfaces ofgrowing insoluble particles and act to keep these particles small;typically less than 1 micron. Thus, the particles are more prone tocontinue to flow through the system and not settle on heat exchanger orother surfaces. This can be described by Stokes law, which is dependenton the radius of the particles. This is schematically shown in FIG. 9.The dispersants act by a combination of steric stabilization, which actsto repel approaching particles (dramatically increase entropy of localsystem and drive solvent in between particles), and blocking of polarsites on the particles which act as a driving force for coalescence.Light scattering evidence exists that shows that dispersant treatedthermally stressed fluids generate particles that are up to two ordersof magnitude smaller than untreated hydrocarbon fluids.

Even if the particles are not small, the above mechanism explains howthe particles will be less prone to coalesce to other particles insolution, or to material already deposited on the surface.

It has also been shown that the nature of the surface plays a role inthe ability of thermally stressed fluids to deposit. Metal surfaces withhigher roughness, edges, or polarity are more prone to fouling. Thesedispersants will adsorb to such surfaces and discourage particulate oramorphous insoluble hydrocarbon from sticking to the surface.

The reaction of hydrocarbons at elevated temperatures with oxygen (evenvery low levels such as <5 ppm) will result in formation of polarfunctionalities that can drive coalescence of particulate, as well asaccelerate the dehydrogenation of adsorbed hydrocarbon, which makes itsremoval from the surface by turbulent flow much less likely. Dispersantadsorption will block the mass transfer of the oxygen to the surface,and some of these described anti-foulants have antioxidant abilities byinterfering with radical reactions.

In addition, the visbroken tar is generally believed to be colloidal innature, with more highly polar and higher molecular weight asphaltenespecies being stabilized in the fluid by smaller resin molecules. As thethermal stress disturbs the relationship of the adsorbed resins toasphaltenes, and by driving the conversion of resins to asphaltenes, andby making the asphaltenes more polar, these systems can be described asbeing more “unstable” or prone to deposition. The dispersants describedhere are believed to replace the disturbed or destroyed resins andre-stabilize the asphaltene system.

As described herein, the VFM measurement data gives information on thesolids content in the residue (tar). Higher amounts of solids will givea higher precipitation potential. The solids might be introduced intothe system by the feed (poor feed quality) and/or through the crackingprocess. The higher the cracking severity the higher the solids contentin the residue likely will be.

Based on defining a baseline, which is unit dependent, the VFM dataprovides information in increasing response to decreasing solids contentin the tar. Depending on the main cause of the solids increase (feed orcracking severity) the device can help to optimize the chemicalinjection rate (if solids are from feed or severity want to bemaintained) in order to maintain the fouling rate and thus keeping unitrun-length under control. If solids increase is due to cracking severityonly, the VFM measurement provides an early warning to potentialinstability of the tar and cracking severity can be reduced bydecreasing the furnace outlet temperature (FOT).

FIG. 10 shows a correlation of FOT versus PV. Increasing the FOT willreduce the PV value up to instability (i.e., PV=1.0). With theappropriate treatment, the PV will remain higher (i.e., stable) at thesame temperature. Also note that the slope between the treated anduntreated curves is different, with the treated curve having a muchgentler slope. This provides more security and flexibility to theconversion enhancement objectives as the treatment acts as a buffer tothe rate of PV change with FOT. Accordingly, FIG. 10 is the correlationof the Furnace Outlet Temperature versus Pv showing that by increasingFOT the Pv will reduce up to instability, and with treatment, the Pvwill be higher at the same temperature, but also the slope is differentindicating that we provide more security and flexibility to theconversion enhancement objectives. By comparison, other known treatmentsystems, such as those described in European Patent Nos. 0321424 B1 and0529397 B1 to Faina, et al., do not impact Pv in the manner described bythe present invention.

Comparing the difference in the VFM measurements from the tar in theinlet of the furnace to measurements in the outlet of the furnace givesa direct measure of the severity of the cracking. When the VFM measuresinhomogeneities in the outlet stream, action can be taken on the processside, specific to customer specifications. For example, the simplestaction to be implemented is reducing the cracking severity in order toreduce the fouling rate on the furnace, exchangers, columns bottoms orsoaker drum. This reduces the risk and rate of fouling deposits, but italso reduces the amount of light hydrocarbon stream produced, so itreduces the profitability of operations. This course of action isaccompanied with the feed of high temperature antifoulant chemistry atthe rate of approximately 100 ppm. In order to maintain the highestefficiency of conversion and therefore the highest profitability, thegoal is to increase the tar stability (increase the P-value) byreplacing the converted resins by high temperature dispersant at ahigher dose that is up to about 500 ppm of chemical is injected. Theeconomical optimum to provide maximum profitability to the refinery isdependent on the individual refinery operations and objectives and islikely on the order of about 300 ppm. The specific value is determinedwith the use of the VFM measurements and our quantitative statisticalmodels.

Our MRA models attempt to define a mathematical correlation between theoperational parameters such as—feed quality, cracking severity,conversion and the fouling rate of the subject exchanger or furnace. Bynormalizing the mathematical model, the fouling rate is isolated fromthe varying operational parameters and the real fouling rate can bedemonstrated and quantified. By developing a corrected model whichreflects the residuals between the predicted model and the actualmeasured parameter, statistical process control techniques may beapplied to quantify the performance of the chemicals applied to controlfouling in the visbreaker unit. Precise determination of the foulingpotential in this manner allows a refinery to start treating anopportunity crude and quickly reach an optimum set of operatingconditions without incurring fouling, or to quickly change furnaceconditions (i.e., temperature) in order to increase or decrease theamount of specific fractions in the product (i.e., distribution ofcomponents and/or composition of visbroken product) which may berequired for immediate production needs, while assuring that operationremains within a safe stability band. In addition to enhanced yield orthroughput, it provides enhanced flexibility with minimized risk.

The present invention is adapted to control chemical feed based on VFMmeasurements to maximize yield of light HC streams in Visbreakeroperations. The VFM can also give an estimate of tar stability, which isproportional to HFT measurements. The program of the present inventioncontrols chemical feed based on a predefined furnace outlet temperature,and uses predictive modeling to verify and predict performance based onVFM measurements. The chemical feed rate is then directly tied tocustomer driven performance measurements such as run length and/orconversion rate. High temperature dispersants can replace the convertedresins to maintain tar stability while increasing the cracking severity;or, the system may increase tar stability by maintaining constantcracking severity. Moreover, measuring the tar characteristics with theVFM before and after the furnace indicates the amount of particulatesproduced directly in the cracking process.

A process for establishing effective visbreaker treatment may besummarized as follows. First, the user clearly defines the problem to besolved. Next, a unit survey or blank test of visbreaker operations isperformed. Next, operational data obtained from the unit survey isanalyzed, and baseline performance parameters are defined. Next,performance goals are measured in accordance with mutually agreed uponproduction goals and requirements, and then an appropriate treatmentprocedure may be designed. Next, the treatment procedure is implemented,monitored and serviced, and finally performance reports and quantitybenefits may be provided.

As shown in FIG. 11, tar stability conversion occurs as asphaltenes aredisbursed in the continuous phase through the peptizing action ofaromatics and resins. It may also be noted from the illustration thatcracking modifies the equilibrium so that asphaltenes could causeprecipitation—low peptisation value.

Exemplary data recorded from a visbreaker conversion trial is shown inFIG. 12. As it is noted from FIG. 12, the circled regions representareas to stop chemical injection under the same operating conditions.

FIGS. 13-16 illustrate exemplary data obtained during conversionenhancement application. As it can be noted from the illustrated data,an overall +3% conversion increase was achieved. In FIG. 15, Thermoflo7R630 was injected before preheat: average 300 ppm. It has to be notedthat even a conversion increase by 1% in the treated charge has to beconsidered extremely satisfying in terms of profit.

FIG. 17 illustrates VFM data versus corrected skin temperature overtime, and FIG. 18 is a schematic diagram illustrating exemplaryvisbreaker process types.

The operation described above of path length sampling to develop ameasure of concentration of dispersed phase correlates well with aconventional HFT measure of hot filtered tar and may also be used with asuitable protocol to derive the classical peptization value Pv. Thisallows the VFM to be used to assess the quality of the visbreakerproduct and efficiently blend or produce various required fuel or otheroils. The classical procedure for measuring Pv, in use for decades,involves slowly adding graded amounts of pure n-cetane C₁₆ H₃₄ to asequence of samples of the product, maintaining each diluted sample in aheated bath for a time (e.g., thirty minutes) to allow the asphaltenesto agglomerate, and then detecting the concentration of tar. Thedifferent samples provide a graph of the product stability, with anabrupt increase in tar separation at the peptization value Pv. Theconcentration measured by the VFM of the present invention provides aneffective tool for performing such a Pv measurement quickly andrepeatably.

One suitable protocol substitute n-heptanes for cetane in the samplepreparation procedure, allowing the dilutions, heating and settling tobe performed quickly—on small samples, at lower temperature, and inshorter times. A classical P value is expressed as 1+Xmin, where Xmin isthe maximum dilution before flocculation occurs expressed in number ofmilliliters of diluent n-cetane per gram of sample. For use with the VFMof the present invention, using n-heptane as the diluent, the sequenceof samples with successively increasing dilution may be heated in awater bath at 100° C. for fifteen minutes, allowed to cool and stand forfifteen minutes, and then measured with the VFM. This substantiallyreduces the sample preparation time, and because the VFM requires only asmall path sampling procedure, the entire array of samples may be placedon a single slide—for example, a 9-well microsample plate, for theconcentration detection step, so measurement is simplified, and madequantifiable and repeatable. Because of the lower molecular weight ofthe lighter heptane diluent, a correction factor 1/0.443 is applied tothe diluent volume Xmin to correct for the different molecular weight ofcetane, so that the resulting P value is identical in value to theclassical measurement. A series of samples can be placed on the stage.Each sample comprises a small amount of aliphatic hydrocarbon (i.e.,n-cetane, n-heptane, etc.). The more aliphatic compound that needs to beadded, the more stable the tar. The light transmission is then measuredover a scan path on each individual sample. This allows a functionalcomparison to be made of optical density to the amount of aliphaticadded to each sample.

FIG. 19A illustrates the derived Pv obtained by this procedure for fivesamples of visbreaker fluid, compared to the P values determined by theclassical n-cetane laboratory testing analysis of the samples. Themeasurements are essentially identical. FIG. 19B graphs the VFMconcentration measurement (in arbitrary units) illustrating the onset ofinstability and flocculation. The value Pv is readily visible as thepoint at which there is a rapid increase in sample opacity with arelatively small increase in the amount of the aliphatic (heptane)diluent. This abrupt change in the VFM concentration measurement amongthe tested samples, may be automatically defined as an output with astraightforward software comparison algorithm to provide thismeasurement of product quality or fluid stability. Other aspects of thesample preparation such as the preparation of a set of differentdilutions and loading onto a microsample array for concentrationmeasurement may be fully automated, using various injection, handlingand transfer mechanisms that will be familiar from similar tasksperformed by equipment used to automate the handling, processing andanalysis of chemical, biological, medical or genetic sequencingmaterials.

Further, the program described above can be adapted to optimize thecontrol of fouling in a heat exchanger network, or other equipment for arefinery process unit (such as a visbreaker unit, coker unit,hydrocracker unit, or crude unit), based on a-priori detection ofparticulate agglomeration, size distribution and insolubility. Morespecifically, fouling caused by asphaltenic instability due to running,or blending, unstable crude diets. The program provides a framework tomonitor the fouling propensity of the unit feedstock with regard tooptimizing blending ratios, and/or controlling a chemical antifoulant ina feed forward manner These techniques provide a way to minimize theenergy and maintenance costs associated with fouling of heat exchangeequipment, as well as to select feed blends that can balance costsavings with fouling control.

It is known that the fouling tendency of hydrocarbon feedstocks, orblends thereof, can be related to the tendency for insoluble organicmaterials to precipitate in heat transfer equipment. The VFM of theinvention measures a quantity relatable to the opaque filterable solidswithin a hydrocarbon sample. The automated program outlined in thepresent invention utilizes VFM concentration and particle sizedistribution data to estimate the fouling potential of a blendedfeedstock. This, in turn, is used to understand the blending envelope ofpotential hydrocarbon feeds, relate it to fouling behavior and then toprovide optimal dosage of chemical treatment.

It is shown that high temperature dispersants and anti-foulants are themain components in a chemical regiment used to treat process equipment,and thereby reduce the rate of fouling. There are specific chemicalfamilies that are particularly effective for use in slowing the rate offouling in heat exchange equipment and other hot surfaces, such asfurnaces. The fouling rates are affected by action of the said chemicalsto colloidally stabilize the contained particulates, includingasphaltenic materials. The program of the present invention isconfigured to allow blending stability envelopes to be ascertained priorto their use. Additionally, the program is configured to select the typeand quantity of chemistry required to satisfy production requirements.Specific chemical entities include, but are not limited to,polyisobutenylphosphonic acids and esters, polyisobutenylthiophosphonicacids and esters, alkylphosphonate phenate sulfides and disulfides thatmay be neutralized with alkaline earth metals, or amines ofpolyisobutenyl succinimides, polyisobutenylsuccinate alkyl esters,magnesium or calcium salts of alkyl or dialkylnaphthelene sulfonic acidsas described in U.S. Pat. No. 4,927,519 and EP Pat. No. 321424B1, hereinincorporated by reference.

These antifoulant materials have been found to function at low dosages,1-200 ppm, to prevent the undesirable deposition of insoluble material(fouling) onto surfaces in heat exchange equipment running processfluids. Fouling in heat exchangers is most generally thought to occur byfirst generating an unstabilized macromolecular particle that is nolonger soluble in the bulk fluid, or as a colloidal species. This oftenoccurs due to thermal stress on the hydrocarbon. Initial depositionoccurs, and further destabilized species adsorb onto the site oforiginal deposition. Bigger particles in the hydrocarbon will be moreprone to contact and coalesce to the surface. Dehydrogenation of theadsorbed hydrocarbon will be driven by heat and time and make thedeposit more tenacious as crosslinking reactions occur.

The dispersants are generally understood to function by a variety ofmechanisms. First, the dispersant materials adsorb to the surfacesformed by growing insoluble particles and act to keep these particlessmall; typically less than 1 micron. Thus, the particles are more proneto continue to flow through the system and not settle on heat exchangeror other surfaces. This can be described by Stokes law, which isdependent on the radius of the particles. This is schematically shown inFIG. 9. The dispersants act by a combination of steric stabilization,which acts to repel approaching particles (dramatically increase entropyof local system and drive solvent in between particles), and blocking ofpolar sites on the particles which act as a driving force forcoalescence. Light scattering evidence exists that shows that dispersanttreated thermally stressed fluids generate particles that are up to twoorders of magnitude smaller than untreated hydrocarbon fluids.

Even if the particles are not small, the above mechanism explains howthe particles will be less prone to coalesce to other particles insolution, or to material already deposited on the surface.

It has also been shown that the nature of the surface plays a role inthe ability of thermally stressed fluids to deposit. Metal surfaces withhigher roughness, edges, or polarity are more prone to fouling. Thesedispersants will adsorb to such surfaces and discourage particulate oramorphous insoluble hydrocarbon from sticking to the surface.

The reaction of hydrocarbons at elevated temperatures with oxygen (evenvery low levels such as <5 ppm) will result in formation of polarfunctionalities that can drive coalescence of particulate, as well asaccelerate the dehydrogenation of adsorbed hydrocarbon, which makes itsremoval from the surface by turbulent flow much less likely. Dispersantadsorption will block the mass transfer of the oxygen to the surface,and some of these described anti-foulants have antioxidant abilities byinterfering with radical reactions.

In addition, hydrocarbon feed stocks, such as crude oils and theirheavier distillate fractions, are thought to contain asphaltenicmaterials that are colloidal in nature. These colloids have highly polarand high molecular weight asphaltene species that are stabilized in thefluid by smaller resin molecules. These systems can be rendered“unstable” or prone to deposition either by thermal stress, whichchanges the critical ratio of the adsorbed resins to asphaltenesrequired to maintain a stable colloidal. The dispersants described hereare believed to replace the disturbed or destroyed resins andre-stabilize the asphaltenic systems.

As described herein, the VFM measurement data provides informationregarding the concentration and particle size distribution of insolublesolids in the hydrocarbon feed and/or its blends and their resultantdistillate fractions. Fluids having higher amounts of solids will give ahigher precipitation potential. Additionally, fluids having particlesize distributions that present substantial fractions greater than acritical size threshold will also exhibit increased fouling potential.The critical size threshold varies, in a practical sense, depending uponmechanical properties of the fluid and its flow, such as viscosity,velocity and shear stress etc. Furthermore, the nature of the particlesmaking up the distribution will also affect the fouling propensity.Finally, the feed, the particular blending ratios of individualfeedstocks comprising the blend, thermal destabilization, anddehydrogenation mechanisms can all introduce solids into the system,affect the size distribution and also and the nature of the particulatematter.

Baseline performance parameters, such as fouling potential, can beestablished by performing a unit survey or blank test of the processunit operations and analyzing the operational data obtained from theunit survey. Based on defining a unit dependent baseline for aparticular feedstock, the VFM data provides information regarding theincreasing or decreasing fouling potential of the incoming feed. Thisinformation can then be used to alter chemical antifoulant injectionrates. Additionally, crude blending practices are often fraught withuncertainty and significant non-linearity can occur with respect toblending ratios when considering the stability of contained asphaltenicmaterials and the associated fouling rates. The VFM data, when runacross the desired blending envelope(s), can define regions ofsignificant fouling potential. This can allow rational decisions to bemade in real-time to adjust blending strategies to minimize foulingand/or maximize run-length.

FIG. 20 and FIG. 21 show VFM particle count data for a particularlyasphaltenic crude oil (Crude A) blended at several ratios with astandard crude slate (Crude B). The chart additionally shows the sameblending curve with the addition of a chemical antifoulant. This set offluids was chosen because this blend has caused severe pre-heat foulingrate increases, as well as desalter upset conditions, on multipleoccasions in the past. Several important points can be noted from thisdata. First is that when considering the pure Crude A and Crude B data,the asphaltenic crude oil (Crude A) has much lower particle count thanthe normal slate (Crude B). This observation shows that the VFM can beused to evaluate incoming crude feeds to ascertain their relativefouling propensity. By performing the VFM test on these crudes after thedesalting process, the intrinsic fouling propensity can be related toasphaltenic components or other inorganic contaminants which can beremoved in the desalting process. Crude feedstocks and blends can beranked as low, medium or high fouling potential feedstocks. Also notablein FIG. 21 is that upon mixing the Crude A and Crude B at varyingratios, the particle count dramatically rises for blend ratios between40% and 60%. This shows that this blend has a tendency to exhibit asignificant non-linear increase in insoluble particulate. This is likelydue to the destabilization of colloidal asphaltenes as outlined above.FIG. 22 shows actual furnace inlet temperature (hereinafter “FIT”)declines due to fouling as measured in the field. The FIT declinedseverely during several timeframes while processing crude diets havingblend ratios in this problematical regime. FIG. 21 also shows that thenumber of insoluble particles can be dramatically affected by theaddition of small amounts of chemical antifoulant. FIG. 23 shows asimilar set of data for another asphaltenic crude (Crude D) blended intoanother standard crude slate (Crude E). The second data set isindicative of fairly widespread experience with these issues and showsthat this is not something particularly related to narrow sets of crudeoils.

The VFM data shown above forms the basis for the current invention inwhich a program is outlined to use the VFM instrument to proactivelycontrol fouling behavior in heat exchange equipment, or other pieces ofprocess equipment prone to fouling, before problematical and costlyimpacts are actually experienced. By using the VFM to screen incomingcrude oil, changes to the incoming particle count can be used to controlchemical feed rates in combination with statistical models. By utilizingthe combination of the VFM, chemical antifoulants and statisticalprocess control methodologies, the overall program can be managed toprovide an improved ability to achieve unit runtime, maintenance andeconomic goals. Additionally, blending strategies can be proactivelydeveloped such that great flexibility to meet overall economic goals isachieved, while preserving critical heat transfer efficiency andruntime.

An exemplary method of the invention is described, wherein the VFMdevice is used to control fouling in a process unit preheat exchangertrain.

A process fluid enters a crude unit after being blended in a tank farm201. It then enters a preheat system composed of a cold exchanger train202, a desalter 203, a hot exchanger train 204, a furnace 205, and afractionating unit 206. Fluid samples are taken at points 207 such thatcharacteristics of the native fluid, before chemical injection, can becompared to the characteristics at various points in the system duringthe heating process and after chemical has been added. Fluid samples arepassed to the VFM 208 and fouling propensity data is determined andcompared at the various sampling points 207. VFM data is thentransmitted to a processing and control mechanism 209 where mathematicalanalysis takes place to determine antifouling chemical demand. Theanalysis of the data consists of using predictive modeling techniques toevaluate the impact of chemical on fouling propensity, as measured bythe VFM, and also to project theoretical run length based on themeasured rate of fouling. Based upon the derived statistical parameters,necessary chemical demand is computed to maintain the fouling ratewithin specified limits. Instructions are then transmitted to thechemical feed pump 210, which delivers chemical from the chemicalstorage tank 211. Based upon the data analysis and control algorithms,an optimized dosage of antifouling chemical is injected by the injectionpump 210 into the process line at injection point 212.

Other schemes can be envisioned, and this embodiment is not meant to belimiting to a specific piece of process equipment, data acquisitionmethodology, feed configuration, or control mechanism. One preferredembodiment might be envisioned to additionally collect data and/orinject chemical prior to tankage when raw process feed is offloaded fromtransportation to the refinery.

Pieces of process equipment at which a scheme similar, but not limitedto, that described above could be envisioned by those skilled in the artare: fractionating unit preheat exchanger trains in the crude unit andhydrotreating unit; FCCU slurry loop exchangers and steam generators;heat exchanger equipment in delayed coking units, furnaces and reboilersthroughout the refinery.

While the disclosure has been illustrated and described in typicalembodiments, it is not intended to be limited to the details shown,since various modifications and substitutions can be made withoutdeparting in any way from the spirit of the present disclosure. As such,further modifications and equivalents of the disclosure herein disclosedmay occur to persons skilled in the art using no more than routineexperimentation, and all such modifications and equivalents are believedto be within the scope and spirit of the disclosure as defined by thefollowing claims.

1. A method to reduce and optimize the fouling rate in a process unitcomprising the steps of: estimating a concentration of foulant materialscontained within a fluid of the process unit; determining the foulingpotential based on said concentration; determining the impact ofantifoulant chemical on the fouling potential within the unit;determining an acceptable baseline fouling potential; comparing saidbaseline parameter to said concentration of foulant materials measuredduring subsequent unit operations; and regulating input of saidanti-fouling chemical into said process unit during running of unitoperations based on said comparison so as to achieve one or more unitgoals.
 2. The method of claim 1, wherein said unit goals includeimproved yield, improved quality, and reduced fouling relatedmaintenance.
 3. The method of claim 2, wherein said estimating stepincludes estimating a distribution of foulant materials contained withinsaid fluid; and said fouling potential is determined by saidconcentration and said distribution.
 4. The method of claim 3, whereinsaid regulating step includes selecting and controlling the type orquantity of said anti-fouling chemical being input into the unit.
 5. Themethod of claim 4, further comprising the step of improving run-lengthscompared to previous un-treated conditions.
 6. The method of claim 5further comprising the use of predictive modeling to derive the impactof chemical on the fouling rate, and the amount of necessary chemical toprovide adequate run length, according to planned maintenance schedules.7. The method of claim 6, further comprising the use of multiplemeasurements, either within the unit or its feed steams, to determinethe impact of introduced chemical upon foulant concentrations.
 8. Themethod of claim 7, further comprising the use of multiple chemicalinjection locations inside and/or outside unit boundaries.
 9. The methodof claim 1, wherein said estimating step is performed using an apparatuscomprising: an optical lens system comprising a stage adapted to receivea sample of fluid; a light sources for focusing a beam of light ontosaid sample; means of directing said light beam along a plurality ofpath lengths within a predetermined area of said sample; means ofdetecting light transmitted through said sample along each path length;means for quantifying an intensity of said transmitted light; and meansfor correlating said quantified transmitted light to a concentration ofsaid foulant particles in said samples.
 10. The method of claim 9,further comprising means for determining the type or quantity ofantifouling chemical injected into the process unit.
 11. The method ofclaim 9, further comprising means for estimating the impact of injectedanti-fouling chemical on foulant particles and run length predictions.12. The method of claim 9, further comprising the usage of measured datato generate predictive models for use in controlling chemical injection.13. The method of claim 9, further comprising the usage of multiplechemical injection locations inside and/or outside the units boundaries.14. A method to improve a hydrocarbon fluid stream in a refinerycomprising the steps of: estimating a concentration of foulant materialscontained within a hydrocarbon fluid stream in a refinery; determiningthe fouling potential based on said concentration; determining theimpact of antifoulant chemical on the foulant materials; determining anacceptable baseline fouling potential; comparing said baseline parameterto said concentration of foulant materials measured during subsequentrefinery operations; and regulating input of said anti-fouling chemicalinto said fluid stream during running of refinery operations based onsaid comparison so as to achieve one or more refinery goals.
 15. Themethod of claim 14, wherein said refinery goals include improved yield,improved quality, improved run-lengths, and reduced fouling relatedmaintenance.
 16. The method of claim 15, wherein said estimating stepincludes estimating a distribution of foulant materials contained withinsaid fluid; and said fouling potential is determined by saidconcentration and said distribution.
 17. The method of claim 16, whereinsaid regulating step includes selecting and controlling the type orquantity of said anti-fouling chemical being input into the fluidstream.
 18. The method of claim 17 further comprising the use ofpredictive modeling to derive the impact of chemical on the foulingrate, and the amount of necessary chemical to provide adequate runlength, according to planned maintenance schedules.
 19. The method ofclaim 18, further comprising: the use of multiple measurements, eitherwithin the refinery or its feed steams, to determine the impact ofintroduced chemical upon foulant concentrations; and the use of multiplechemical injection locations.
 20. The method of claim 19, wherein saidestimating step is performed using an apparatus comprising: an opticallens system comprising a stage adapted to receive a sample of fluid; alight sources for focusing a beam of light onto said sample; means ofdirecting said light beam along a plurality of path lengths within apredetermined area of said sample; means of detecting light transmittedthrough said sample along each path length; means for quantifying anintensity of said transmitted light; and means for correlating saidquantified transmitted light to a concentration of said foulantparticles in said samples.
 21. A system to reduce and optimize thefouling rate in a process unit comprising: an optical lens systemcomprising a stage adapted to receive a sample of fluid; a light sourcesfor focusing a beam of light onto said sample; means of directing saidlight beam along a plurality of path lengths within a predetermined areaof said sample; means of detecting light transmitted through said samplealong each path length; means for quantifying an intensity of saidtransmitted light; and means for correlating said quantified transmittedlight to a concentration of said foulant particles in said samples.means for determining the fouling potential based on said concentration;means for determining the impact of antifoulant chemical on the foulingpotential within the unit; means for determining an acceptable baselinefouling potential; means for comparing said baseline parameter to saidconcentration of foulant materials measured during subsequent unitoperations; and means for regulating input of said anti-fouling chemicalinto said process unit during running of unit operations based on saidcomparison so as to achieve one or more unit goals.